A retardation element has a function of converting the direction of linearly polarized light vibrating in a certain direction to another direction, or a function of converting circularly polarized light or elliptically polarized light to linearly polarized light, and by utilizing these functions, for example, viewing angles, a contrast, and the like of liquid crystal displays (LCD) can be improved. As specific examples of utilizing this function of converting polarized light of the retardation element, there are known an example in which a retardation element referred to as a half-wave plate is employed as a polarization rotator of a polarizing beam splitter of a liquid crystal projector, as disclosed in Patent Document 1, and an example in which a retardation element referred to as a quarter-wave plate is employed in order to convert circularly polarized light obtained from a cholesteric liquid crystal to linearly polarized light, as disclosed in Patent Document 2. This quarter-wave plate is known to be used for an anti-reflection filter or the like as a circularly polarizing plate formed by being combined with a polarizing plate.
Examples of the retardation element include retardation elements formed by uniaxially stretching or biaxially stretching conventionally known plastic films such as polycarbonate films, polyarylate films, polyethersulfone films, cycloolefin polymer films, and the like. Generally, these retardation elements are referred to as a retardation plate or a retardation film.
Performance of the retardation element can be determined, for example, by a retardation value derived from the product of the birefringence which represents the difference between the refractive index in a direction of a slow axis (a direction in which the refractive index is maximized in one plane) and the refractive index in a direction of a fast axis (a direction perpendicular in one plane to the slow axis direction) and the thickness of the element. Further, in recent years, as disclosed in Patent Documents 3 to 7, the retardation element has been prepared by aligning a liquid crystalline compound in a specific direction and fixing the alignment state.
The retardation element using the liquid crystalline compound has been attracting attention since the element is characterized in that it can be a thin film which cannot be realized in a plastic film and can realize a complicated alignment state which cannot be realized by stretching a plastic film.
It is known that viewing angle properties, colors and a contrast of various liquid crystal displays can be improved by using the retardation element employing such a liquid crystalline compound. For example, Patent Document 3 discloses an improvement in viewing angle properties of a TN (twisted nematic) type liquid crystal display which uses a retardation element having a discotic liquid crystal layer in which a hybrid alignment is employed. Further, in Patent Document 4, color compensation of a STN (super twisted nematic) type liquid crystal display is disclosed. Patent Document 5 discloses an improvement in viewing angle of an ECB (electrically controlled birefringence) type liquid crystal display which uses a retardation element having a liquid crystalline polyester in which a hybrid alignment is employed. Furthermore, Patent Document 6 discloses an improvement in viewing angle properties of a VA (vertically alignment) type liquid crystal display which uses a twisted aligned liquid crystal layer having a wavelength region of selective reflection in an ultraviolet region and an OCB (optically compensated bond) type liquid crystal display. In Patent Document 7, compounds to be used for the retardation element, in which liquid crystalline compounds for compensating the above-mentioned liquid crystal displays are used, and a production method are disclosed.
The above-mentioned retardation value is important in order that the retardation element properly convert the polarized light at a desired wavelength.
However, the retardation value can vary under various environmental conditions, for example, in an atmosphere of high temperature or in an atmosphere of high temperature and high humidity, and the solution of the problem is desired. For example, in the case of liquid crystal displays used in an automobile instrument panel section or a liquid crystal projector, the viewing angle properties or the contrast of the liquid crystal display may be deteriorated when the retardation value varies due to exposure of the display to a high temperature atmosphere.